Rainbow Electronics RF22 - Manual (Page 28)

RF22
Version: 0.1 Date: 12/23/2008
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Preamble SYNC
HEADER/
ADDRESS
PK
Length
DATA CRC
CRC
Whitening
Manchester
CRC (data only = 1)
6.6. Preamble Detector
The RF22 has integrated automatic preamble detection. The preamble length is configurable from 1–256 bytes using
the prealen[7:0] field in "Register 33h. Header Control 2" and "Register 34h. Preamble Length", as described in “6.2.
Packet Configuration”. The preamble detection threshold, preath[4:0] as set in "Register 35h. Preamble Detection
Control 1” is in units of 4 bits. The preamble detector searches for a preamble pattern with a length of preath[4:0].
When a false preamble detect occurs, the receiver will continuing searching for the preamble when no sync word is
detected.
The Preamble Detector output may be programmed onto one of the GPIOs or read in the Interrupt Status registers.
6.7. Preamble Length
The required preamble length threshold will depend on when the receive mode is entered in relation to the transmitted
packet. When the receiver is enabled long before the arrival of the packet, then a short preamble detection threshold
might result in false detects on the received noise before the actual preamble arrives. In this case, it is recommended
to program a 16 or 20 bit detection threshold. When antenna diversity is enabled, it is advised to use the default
preamble detection threshold (16 bit). When the receiver is synchronously enabled just before the start of the packet,
then a shorter preamble detection threshold might be chosen (e.g., 8 bit).
The required preamble length is determined from the sum of the receiver settling time and the preamble detection
threshold. The receiver settling time is listed in Table 15.
Table15. Minimum Receiver Settling Time
Mode
Approximate receiver
settling time
Recommended preamble
length with 8-bit detection
threshold
Recommended preamble
length with 16-bit detection
threshold
(G)FSK AFC Disabled 1 byte 2 byte 3 byte
(G)FSK AFC Enabled 2 byte 3 byte 4 byte
(G)FSK AFC Disabled +Antenna Diversity
Enabled
1 byte — 7 byte
(G)FSK AFC Enabled +Antenna Diversity
Enabled
2 byte — 8 byte
OOK 2 byte 3 byte 4 byte
OOK + Antenna Diversity Enabled 8 byte — 8 byte
Note: The recommended preamble length may be shortened when occasional packet errors are allowed.
6.8. Invalid Preamble Detector
When scanning channels in a Frequency Hopping System, it is desirable to determine if a channel is valid in the
minimum amount of time. The preamble detector can output an invalid preamble detect signal. When an error is
detected in the preamble, the Invalid Preamble Detect signal (nPQD) is asserted, indicating an invalid channel. The
signal can be used to qualify the channel without requiring the full preamble to be received. The Preamble Detect and
Invalid Preamble Detect signals are available in "Register 03h. Interrupt/Status 1" and "Register 04h. Interrupt/ Status
2".